Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Class A common stock	Class B common stock	Class U common stock	Common Stock Class A common stock	Common Stock Class B common stock	Common Stock Class U common stock	Treasury Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balance, Beginning at Dec. 31, 2017	$ 348,275				$ 7	$ 2	$ 1		$ 888,650	$ (540,325)	$ (60)
Balance, Beginning, Shares at Dec. 31, 2017					66,069,325	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					17,356
Tax payments related to shares withheld for share-based compensation plans	(43)								(43)
Stock-based compensation expense	1,249								1,249
Repurchase of Class A common stock	(2,402)								(2,402)
Repurchase of Class A common stock, Shares					(500,587)			500,587
Retirement of treasury stock, Shares								(500,587)
Dividends paid	(4,518)								(4,518)
Change in fair value of marketable securities	(1,239)										(1,239)
Foreign currency translation gain (loss)	290										290
Net income (loss)	(1,808)									(1,808)
Balance, Ending at Mar. 31, 2018	339,804				$ 7	$ 2	$ 1		882,936	(542,133)	(1,009)
Balance, Ending, Shares at Mar. 31, 2018					65,586,094	14,927,613	9,352,729
Balance, Beginning at Dec. 31, 2017	348,275				$ 7	$ 2	$ 1		888,650	(540,325)	(60)
Balance, Beginning, Shares at Dec. 31, 2017					66,069,325	14,927,613	9,352,729
Foreign currency translation gain (loss)	(220)
Net income (loss)	5,248
Balance, Ending at Sep. 30, 2018	334,833				$ 6	$ 2	$ 1		871,321	(535,078)	(1,419)
Balance, Ending, Shares at Sep. 30, 2018					64,576,497	14,927,613	9,352,729
Balance, Beginning at Mar. 31, 2018	339,804				$ 7	$ 2	$ 1		882,936	(542,133)	(1,009)
Balance, Beginning, Shares at Mar. 31, 2018					65,586,094	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	106								106
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					56,147
Tax payments related to shares withheld for share-based compensation plans	(12)								(12)
Stock-based compensation expense	1,176								1,176
Repurchase of Class A common stock	(5,258)				$ (1)				(5,257)
Repurchase of Class A common stock, Shares					(1,088,086)			1,088,086
Retirement of treasury stock, Shares								(1,088,086)
Dividends paid	(4,441)								(4,441)
Change in fair value of marketable securities	(3)										(3)
Foreign currency translation gain (loss)	(550)										(550)
Net income (loss)	4,840									4,840
Balance, Ending at Jun. 30, 2018	335,662				$ 6	$ 2	$ 1		874,508	(537,293)	(1,562)
Balance, Ending, Shares at Jun. 30, 2018					64,554,155	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	(29)								(29)
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					22,342
Stock-based compensation expense	1,286								1,286
Dividends paid	(4,444)								(4,444)
Change in fair value of marketable securities	103										103
Foreign currency translation gain (loss)	40										40
Net income (loss)	2,215									2,215
Balance, Ending at Sep. 30, 2018	334,833				$ 6	$ 2	$ 1		871,321	(535,078)	(1,419)
Balance, Ending, Shares at Sep. 30, 2018					64,576,497	14,927,613	9,352,729
Balance, Beginning at Dec. 31, 2018	332,732				$ 6	$ 2	$ 1		862,299	(528,164)	(1,412)
Balance, Beginning, Shares at Dec. 31, 2018		63,210,531	14,927,613	9,352,729	63,210,531	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	(42)								(42)
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					25,059
Stock-based compensation expense	800								800
Repurchase of Class A common stock	(7,706)								(7,706)
Repurchase of Class A common stock, Shares					(2,098,443)			2,098,443
Retirement of treasury stock, Shares								(2,098,443)
Dividends paid	(4,271)								(4,271)
Change in fair value of marketable securities	763										763
Foreign currency translation gain (loss)	30										30
Net income (loss)	1,424									1,424
Balance, Ending at Mar. 31, 2019	323,730				$ 6	$ 2	$ 1		851,080	(526,740)	(619)
Balance, Ending, Shares at Mar. 31, 2019					61,137,147	14,927,613	9,352,729
Balance, Beginning at Dec. 31, 2018	332,732				$ 6	$ 2	$ 1		862,299	(528,164)	(1,412)
Balance, Beginning, Shares at Dec. 31, 2018		63,210,531	14,927,613	9,352,729	63,210,531	14,927,613	9,352,729
Foreign currency translation gain (loss)	(392)
Net income (loss)	(27,072)
Balance, Ending at Sep. 30, 2019	285,976				$ 6	$ 2	$ 1		841,567	(555,236)	(364)
Balance, Ending, Shares at Sep. 30, 2019		60,268,985	14,927,613	9,352,729	60,268,985	14,927,613	9,352,729
Balance, Beginning at Mar. 31, 2019	323,730				$ 6	$ 2	$ 1		851,080	(526,740)	(619)
Balance, Beginning, Shares at Mar. 31, 2019					61,137,147	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					20,000
Stock-based compensation expense	836								836
Repurchase of Class A common stock	(1,302)								(1,302)
Repurchase of Class A common stock, Shares					(438,534)			438,534
Retirement of treasury stock, Shares								(438,534)
Dividends paid	(4,269)								(4,269)
Change in fair value of marketable securities	523										523
Foreign currency translation gain (loss)	(318)										(318)
Net income (loss)	(16,279)									(16,279)
Balance, Ending at Jun. 30, 2019	302,921				$ 6	$ 2	$ 1		846,345	(543,019)	(414)
Balance, Ending, Shares at Jun. 30, 2019					60,718,613	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	(21)								(21)
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					21,410
Stock-based compensation expense	819								819
Repurchase of Class A common stock	(1,349)	$ (1,300)							(1,349)
Repurchase of Class A common stock, Shares		(500,000)			(471,038)			471,038
Retirement of treasury stock, Shares								(471,038)
Dividends paid	(4,227)								(4,227)
Change in fair value of marketable securities	154										154
Foreign currency translation gain (loss)	(104)										(104)
Net income (loss)	(12,217)									(12,217)
Balance, Ending at Sep. 30, 2019	$ 285,976				$ 6	$ 2	$ 1		$ 841,567	$ (555,236)	$ (364)
Balance, Ending, Shares at Sep. 30, 2019		60,268,985	14,927,613	9,352,729	60,268,985	14,927,613	9,352,729